Materials exhibiting high D.C. volume resistivity coupled with low A.C. dielectric losses, e.g., steatite, forsterite, and very high alumina-containing bodies, have found extensive use in such applications as electrical insulators and substrate materials. For the former application a very low loss tangent is demanded. The above-recited commercial materials are sintered ceramics with the disadvantageous characteristics inherent in such bodies as, e.g., porosity and surfaces which are not completely smooth. Moreover, the configurations of the products and the rate of production thereof are limited by the sintering technique of manufacture.
Glass-ceramic articles are prepared via the heat treatment of precursor glass bodies. Accordingly, glass-ceramics can be produced utilizing forming methods conventional in the glassmaking art, e.g., blowing, casting, drawing, pressing, rolling, etc. Furthermore, as is well recognized, glass articles have smooth surfaces and are non-porous.
U.S. Pat. No. 2,920,971 constitutes the basic disclosure in the field of glass-ceramics and reference is made to that patent for further discussion of the microstructure and properties thereof, as well as the manner in which such articles are prepared. In brief, however, glass-ceramics are produced in accordance with three general steps. First, a glass-forming composition, which customarily also contains a nucleating agent, is melted. Second, the melt is simultaneously cooled to a glass sufficiently quickly to prevent the occurrence of any substantial crystallization and an article of a desired geometry is shaped therefrom. Third, the glass is subjected to a heat treatment to cause the growth of crystals in situ. Commonly, the crystallization step will be carried out in two parts. Such involves initially heating the glass shape to a temperature within or somewhat above the transformation range thereof to cause the development of nuclei in the glass. After the nucleating step, the glass shape is heated to a higher temperature, commonly above the softening point thereof, to effect the growth of crystals on the nuclei.
Because the growth of crystals takes place on nuclei dispersed throughout the glass shape, a glass-ceramic will typically have a microstructure of fine-grained crystals randomly oriented, but uniformly dispersed, throughout a residual glassy matrix. In general, a glass-ceramic is predominantly crystalline, i.e., greater than 50% by volume crystalline, such that the physical and chemical properties exhibited thereby will be more comparable to the crystal phase present therein than to the properties of the original glass. Moreover, the composition of the small amount of residual glass will be quite different from that of the original glass inasmuch as the components making up the crystals will have been removed therefrom. Finally, because glass-ceramics are prepared via the crystallization in situ of glass articles, they will have smooth surfaces and be non-porous.
Anorthite, i.e., triclinic CaO.Al.sub.2 O.sub.3.2SiO.sub.2, has been known for its insulating properties. However, those properties are not sufficiently different from those of steatite and forsterite to warrant the added expense of manufacture. Nevertheless, it has been realized that a glass-ceramic body wherein the predominant and, preferably, the sole crystal phase is anorthite could have significant practical application. Such bodies would have high dielectric constants and D.C. volume resistivities coupled with low loss tangents and, consequently, would be competitive with commercial electrically-insulating materials. Because of their mode of manufacture, such bodies could be produced at a rapid rate and would have the advantages of smooth surfaces and no porosity.
Anorthite-containing glass-ceramics have been known to the art. For example, U.S. Pat. No. 2,920,971, supra, reports several exemplary compositions wherein the primary crystal phase is anorthite. Nevertheless, whereas bodies of fine-grained, well-crystallized microstructure have been prepared, such have commonly been plagued with a deleterious surface crystallization mechanism which causes a badly-distorted, wrinkled surface appearance. Close examination of this surface crystallization has frequently determined the presence of a high expansion, hexagonal feldspar form of CaO.Al.sub.2 O.sub.3.2SiO.sub.2 rather than the desired triclinic anorthite.
U.S. Pat. No. 3,531,303 describes the production of alkaline earth aluminosilicate glass-ceramic articles from widely-varying base compositions wherein a hexagonal alkaline earth feldspar constitutes the predominant crystal phase.